This invention is concerned with processes employing iodine-iodide etching solutions in the recovery of precious metals. More particularly, the invention is concerned with the use of two aqueous iodine-iodide etching solutions, comprising 1) hydriodic acid and iodine, or 2) ammonium iodide and iodine, for recovering precious metals; with the treatment of the used etching solutions to permit their reuse and/or to prepare new etching solutions from spent solutions; and with the use of hydrogen peroxide or equivalents for accelerating the etching of gold and other precious metals.
Processes employing iodine-iodide etching solutions for recovery of precious metals from ores or scrap material were proposed at least as early as 1907. The number of patents disclosing such processes is substantial. In recent years, iodine-iodide etching solutions, such as an aqueous solution of iodine and potassium iodide, have been employed in the electronics and jewelry industries for etching or stripping gold and other precious metals. Iodine-iodide etching solutions avoid the severe toxicity and waste disposal problems associated with cyanide etching solutions, but higher cost of processes using iodine-iodide etching solutions has limited their use.
In U.S. Pat. No. 3,957,505, an attempt was made to reduce the cost of precious metal recovery using an iodine, potassium iodide etching solution, by regenerating the spent etching solution and reusing it repetitively. However, the effectiveness of the etching solution is appreciably diminished by the presence and accumulation of interfering materials, such as salts of base metals. Furthermore, when, after repeated reuse-regeneration cycles, iodine is extracted for use in forming a new etching solution, it is still necessary to provide new potassium iodide, which is costly (the ratio of iodide to iodine in the solution is ordinarily at least 4:1, and may range from 3 to 7 parts iodide to 1 part iodine).
Co-pending application Ser. No. 610,181, filed May 14, 1984, now abandoned (which is a continuation of Ser. No. 456,285, filed Jan. 6, 1983, now abandoned), incorporated herein by reference, discloses significantly improved processes employing iodine-iodide etching solutions in the recovery of precious metals. The processes of the invention of the co-pending application are simpler, more efficient, and less expensive than comparable processes of the prior art. In one of its broader aspects, the invention of the co-pending application provides, in a process employing an etching solution comprising iodine and iodide for the recovery of precious metal, an improvement in which a new etching solution comprising both iodine and iodide is prepared from spent etching solution by treating the spent etching solution to precipitate out, as elemental iodine, substantially all of the iodine in the iodine-iodide etching solution (in whatever form) and employing the elemental iodine to supply both the iodine and the iodide in the new etching solution. The new etching solution is formed without introducing ions foreign to the original etching solution and without producing undesirable by-products.